This invention relates to a common mode rejection coupler for electrically isolating the input of an electrical system from its output.
The difficulty of coupling a low level signal to a circuit having a transient high level common mode voltage is known in the art. In high voltage or high power switching systems, a low level input logic control circuit provides control signals for an output high level power switch control circuit. Typically, the reference potential of the high level output circuit changes by several hundred volts with respect to the ground potential of the low level control circuit when switching occurs. The potential difference between the floating reference of the high level switching circuit and the ground of the low level control system is termed the common mode voltage. Among techniques known in the art for effectively rejecting common mode voltages while at the same time effectively utilizing low level signals in systems having high common mode voltages include the use of magnetically coupled current transformers, differential amplifiers and various systems which convert the low level signal into another form of signal which can be controlled or monitored at some remote point. The circuits have some limitations particularly because of low frequency response in the case of a transformer coupling, the inapplicability of differential amplifiers to isolate high common mode voltages, and the lack of flexibility when utilizing other systems known in the art.
More recently, as disclosed in Broderick et al., U.S. Pat. No. 3,978,343, and in Loucks, U.S. Pat. No. 4,032,843, optically coupled isolators have been utilized to reject common mode voltages. In its basic form an optically coupled isolator includes a light emitting diode or similar current-to-light transducer which generates light having an intensity which is proportional to the current. The light thus generated is directed to a photosensitive device via an optical path which typically is in the form of a fiber optic bundle. The photosensitive device in turn generates a current which is proportional to the light intensity. Since the optical path is inherently an insulating medium of very high resistivity, such a device has the capability of isolating rather high common mode voltages. Accordingly, these devices have been utilized in solid state switching systems particularly where a low level logic control circuit controls a high power switching circuit.
However, one of the problems encountered with the prior art optically coupled isolator circuits is that the higher frequency common mode components, i.e., the frequency components generated when the common mode voltage rapidly switches from a first level to a second level, can be capacitively coupled via the stray capacitance between the light emitting diode and the light sensing diode in the electrical circuit. Accordingly, this rapid transition in the common mode voltage can result in the generation of undesirable spurious or transient signals by the optical isolator which can then be passed through the system to effect the operation of the isolated switching circuitry.
In the example of power switching systems, as aforementioned, the power switching circuitry is controlled by low level logic control circuitry. The reference potential of the high power switching circuitry floats with respect to the low level logic control circuitry such that the common mode voltage can vary in the range of .+-.100 to 500 volts in a few microseconds. Because of these high voltage transitions, the stray capacitance between the light emitting diode and the light sensitive diode causes a signal corresponding to the change in common mode voltage, i.e., the voltage differential, to be coupled to the high power switching circuitry which might therefore cause faulty switching of the high power switching circuitry. There accordingly is a need in the art for an improved common mode rejection coupler to eliminate spurious signals resulting from transitions in the common mode voltage.
Copending U.S. patent application S.N. filed concurrently herewith and which is assigned to the common assignee herewith discloses one such system for rejecting spurious or transient signals caused by rapid changes in the common mode voltage.